A secondary battery is a battery that is able to convert chemical energy into electrical energy and discharge the energy. Moreover, it is also a battery that is able to convert electrical energy into chemical energy and store (charge) the chemical energy, by passing electrical current in a direction that is opposite to the direction of current at the time of discharge. Of secondary batteries, a lithium secondary battery has high energy density, so that it is widely used as a power source for portable devices such as a laptop personal computer, a cellular phones etc.
In a lithium secondary battery, when graphite (C) is used as an anode active material, a reaction described by the following formula (I) proceeds at the anode, upon discharge:LixC→C+xLi++xe−  (I)wherein 0<x<1.
Electrons generated by the reaction of the formula (I) pass through an external circuit, work by an external load, and then reach the cathode. Lithium ions (Li+) generated by the reaction of the formula (I) are transferred by electro-osmosis from the anode side to the cathode side through an electrolyte sandwiched between the anode and the cathode.
When lithium cobaltate (Li1−xCoO2) is used as a cathode active material, a reaction described by the following formula (II) proceeds at the cathode, upon discharge:Li1−xCoO2+xLi++xe−→LiCoO2  (II)wherein 0<x<1.
Upon charging the battery, reactions which are reverse to the reactions described by the above formulae (I) and (II) proceed at the anode and the cathode. At the anode, graphite in which lithium has been intercalated by graphite intercalation (LixC) becomes reusable, while lithium cobaltate (Li1−xCoO2) is regenerated at the cathode. Because of this, discharge becomes possible again.
Conventional lithium secondary batteries are limited in the improvement of reliability, since they use volatile organic solvents.
Meanwhile, a lithium secondary battery that uses an ionic liquid (room-temperature molten-salt) as liquid electrolyte has been known as an effort to improve reliability. “Ionic liquid” as used herein is a salt which is liquid at 100° C. or less, and it is generally non-volatile. Such a liquid electrolyte is advantageous in that it can not only improve reliability but also shows a relatively wide potential window (potential range) and provides a relatively high ion conductivity.
As a lithium secondary battery technique comprising an ionic liquid, a non-aqueous electrolyte secondary battery technique is disclosed in Patent Literature 1, which comprises a cathode, an anode and a non-aqueous electrolyte containing an ionic liquid and allyl phosphate.
In recent years, tetrazolium mesoionic compounds have received attention, due to their wide potential window and low melting point. A tetrazolium mesoionic compound-related technique is disclosed in Patent Literature 2, which has an alkyl or aryl group at the 1-position and an alkyl group at the 3-position.